1. to improve negotiations when he sues for peace
2. to prevent Ukraine from staging a counterattack
1.improve negotiations with whom? Does he think he’s winning hearts and minds by blowing up homes that women thought they were evacuating for a few days but who lost everything, including husbands, fathers and sons, because of the advance? That the women will return and be content with Russian families moving into their homes and taking their businesses? Does he think that the West will say, ok, you took a country, fine, have a nice day?
2. At best, Russia is present in 5% of the country - all periphery. That leaves a lot of country with friendly borders from which to mobilize relentless counterattacks from many directions by people with nothing left to lose - ala Wolverines.
He might have gotten away with taking the two disputed areas as DMZs without much side-eye from the world, but the entire country? Not seeing a victory for him here, politically or militarily. Rather, more of another Afghanistan death zone - only on steroids times 10.